1. Field of Invention
The present invention relates to electrochemical fuel cells and more particularly to a reactant supply apparatus for a fuel cell, a fuel cell and fuel cell stack employing the same.
2. Description of Related Art
Electrochemical fuel cells convert fuel and an oxidant to electricity and a reaction product. A typical fuel cell includes a cathode, an anode, and a membrane. The membrane is sandwiched between the cathode and anode. Fuel, in the form of hydrogen, is supplied to the anode where a catalyst, usually platinum, catalyzes the following anode reaction.Anode reaction: H2→2H++2ē
Hydrogen separates into hydrogen ions and electrons. The hydrogen (cations) migrate through the membrane to the cathode. The electrons migrate via an external circuit in the form of electricity.
An oxidant, such as pure oxygen or air containing oxygen, is supplied to the cathode where it reacts with the hydrogen ions that have crossed the membrane and with the electrons from the external circuit to form liquid water as the reaction product. The cathode reaction is also usually catalyzed by platinum and occurs as follows.Cathode reaction: ½O2+2H++2ē→H2O
Thus the fuel cell generates electricity and water through the electrochemical reaction. Water is formed at the cathode.
Typically, the electrochemical reaction also supports a phenomenon called water pumping. As each cation (proton) migrates through the membrane, it transports or drags along several water molecules with it. Thus, there is a net transport of water to the cathode. Water pumping adds water to the product water formed at the cathode as a result of the electrochemical reaction in the fuel cell.
Solid polymer fuel cells generally comprise a Membrane-Electrode Assembly (MEA). The MEA consists of a solid polymer electrolyte or ion exchange membrane situated between and in contact with two electrodes, made of porous, electrically conducting sheet material, which act as the anode and cathode. The electrodes are typically made from carbon fiber paper or cloth. At the interface of the electrode and membrane is a layer of catalyst to facilitate the electrochemical reaction. The MEA is placed between two electrically conductive plates, commonly formed from graphite. These plates have one or more reactant flow passages impressed on their surfaces. The reactant flow passages direct the flow of a reactant to the electrode and carry away water produced at the cathode due to the fuel cell reaction and due to water pumping.
Conventional reactant flow passages are generally long, narrow and serpentine in shape. Typically, due to capillary action, water adheres to walls of the reactant flow passages, requiring considerable pressure to remove it. Failure to remove this water can result in the accumulation of water at the cathode, and this can create problems for the operation of the fuel cell. The presence of water in the vicinity of the catalyst layer reduces the accessibility of the catalyst to the reactant, a phenomenon commonly referred to as “flooding.” Also, the presence of water, often in the form of droplets, can substantially block the flow of oxidant reactant through the reactant flow passages. “Dead spots” can form in areas where channel passages are blocked. In addition, the failure to remove water from the cathode can result in localized hot spots in the membrane as the removal of water is important to cooling the fuel cell. Localized hot spots can result in pinhole failure of the membrane, for example. These conditions can result in a reduction of available power from the fuel cell, or failure in operation of the fuel cell.